Conventional flexible polyurethane foams are widely known. Such foams show a relatively high resilience (ball rebound), a relatively low modulus, a relatively high sag factor and a relatively low hysteresis loss. Such foams further show a major glass-rubber transition below ambient temperature, generally in the temperature range of -100.degree. C. to -10.degree. C. The commonly applied relatively high molecular weight polyether and polyester polyols in such foams are responsible for the sub-ambient glass transition temperature (Tg.sup.s). These polyether and polyester polyols are often referred to as soft segments. Above Tg.sup.s the foam displays its typical flexible properties until softening and/or melting of the isocyanate-derived urethane/urea clusters ("hard domains") takes place. This softening and/or melting temperature (Tg.sup.h and/or Tm.sup.h) often coincides with the onset of thermal degradation of polymer segments. The Tg.sup.h and/or Tm.sup.h for flexible polyurethane foams is generally higher than 100.degree. C., often even exceeding 200.degree. C. At the Tg.sup.s a sharp decrease of the modulus of the flexible foam is observed. Between Tg.sup.s and Tg.sup.h /Tm.sup.h the modulus remains fairly constant with increasing temperature and at Tg.sup.h /Tm.sup.h again a substantial decrease of the modulus may take place. A way of expressing the presence of Tg.sup.s is to determine the ratio of the Young's storage modulus E' at -100.degree. C. and +25.degree. C. as per Dynamic Mechanical Thermal Analysis (DMTA measured according to ISO/DIS 6721-5). For conventional flexible polyurethane foams the ##EQU1## is at least 25. Another feature of Tg.sup.s by DMTA (ISO/DIS 6721-5) is that for conventional flexible polyurethane foams the maximum value of the ratio of ##EQU2## over the -100.degree. C./+25.degree. C. temperature range varies from 0.20-0.80 in general. The Young's loss modulus E" is measured by DMTA (ISO/DIS 6721-5) as well.
Conventional flexible foams are made by reacting a polyisocyanate and a relatively high molecular weight isocyanate reactive polymer, often a polyester or polyether polyol, in the presence of a blowing agent and optionally further using limited amounts of relatively low molecular weight chain extenders and cross-linkers and optionally using additives like catalysts, surfactants, fire retardants, stabilisers and antioxidants. The relatively high molecular weight isocyanate reactive polymer in general represents the highest weight fraction of the foam. Such flexible foams may be prepared according to the one-shot, the quasi- or semi-prepolymer or the prepolymer process. Such flexible foams may be moulded foams or slabstock foams and may be used as cushioning material in furniture and automotive seating and in mattresses, as carpet backing, as hydrophilic foam in diapers and as packaging foam. Further they may be used for acoustic applications, e.g. sound insulation. Examples of prior art for these conventional flexible foams are EP-10850, EP-22617, EP-111121, EP-296449, EP-309217, EP-309218, EP-392788 and EP-442631.
Conventional rigid foams are made in a similar way with the proviso that often the polyisocyanates have a higher isocyanate functionality, the amount of high molecular weight polyols used is lower and the amount and functionality of the cross-linkers is higher.
WO92/12197 discloses an energy-absorbing, open-celled, water-blown, rigid polyurethane foam obtained by reacting a polyurethane foamformulation, comprising water which act as a blowing agent and a cell-opener, in a mould wherein the cured foam has a moulded density of about 32 to 72 kg/m.sup.3 and a crush strength which remains constant from 10 to 70% deflection at loads of less than 70 psi. The foams have minimal spring back or hysteresis.
GB2096616 discloses a directionally flexibilized, rigid, closed-cell plastic foam. The rigid foams are flexibilized in order to use them for e.g. pipe-insulation. Cells should remain closed.
U.S. Pat. No. 4,299,883 discloses a sound-absorbent material made by compressing a foam having closed cells to such an extent that the foam recovers to 50-66% of its original thickness. By the compression the cells are ruptured and the foam becomes flexible and resilient; it may replace felt. The disclosure mainly refers to polycarbodiimide foams.
EP561216 discloses the preparation of foam boards having improved heat insulation properties, wherein the foam has anisotropic cells having a length ratio of the long and the small axis of 1.2-1.6 and a density of 15-45 kg/m.sup.3 and wherein the cells have been crushed in the direction of the plate thickness. The disclosure actually refers to polystyrene boards.
EP641635 discloses a process for preparing foam boards, having a dynamic stiffness of at most 10 MN/n.sup.3, by crushing a board of 17-30 kg/m.sup.3 density at least twice to 60-90% of its original thickness. Preferably closed-celled polystyrene is used. In the examples it is shown that a polystyrene foam which has been crushed showed a better heat insulation than an uncrushed one.
U.S. Pat. No. 4,454,248 discloses a process for preparing a rigid polyurethane foam wherein a partially cured rigid foam is softened, then crushed and re-expanded and fully cured.
In copending patent application PCT/EP9601594 a completely new class of flexible polyurethane foams is described such foams having no major glass-rubber transition between -100.degree. C. and +25.degree. C. In more quantitative terms these foams show a ratio E'.sub.-100.degree. C. /E'.sub.+25.degree. C. of 1.3 to 15.0, preferably of 1.5 to 10 and most preferably of 1.5 to 7.5. The tan.sub..delta.max over the -100.degree. C. to +25.degree. C. temperature range is below 0.2.
The apparent core density of such foams may range from 4-30 kg/m.sup.3 and preferably ranges from 4-20 kg/m.sup.3 (measured according to ISO/DIS845). Such foams are made by crushing a rigid foam.
In said co-pending patent application it is stated that due to the crushing the density of the foam may increase; such increase in generally will not exceed 30% of the density before crushing. In the examples density increases have been reported of 18, 15 and 12.5%.
Surprisingly it has now been found that by giving these foams a heat-treatment after they have been crushed the density increase can be further limited.